1. Field of the Invention
The present invention relates to an image forming apparatus provided with a function for detecting a toner marker on a transfer member supporting a toner image transferred from a photosensitive member, and specifically relates to multi-color overlay position dislocation in a color image forming apparatus.
2. Description of the Related Art
In conventional color image forming apparatuses, the density difference between a transfer member and various color toner marks transferred onto the belt or drum-like transfer member is detected via a photosensor, and the positional dislocation of the toner marks is detected by the output of the photosensors.
In such conventional apparatuses, however, photosensors generally detect the density difference between the transfer member and toner marks created by each process for colors Y, M, C, K, the center position of the mark is determined from the photosensor output to infer the position of each mark, and the degree of dislocation if any of the position of marks of each color from a standard position is discriminated and the amount of such dislocation is corrected during image formation. Essentially, although determination of the image overlay condition is best when the images have been overlaid, it is necessary to determine the dislocation from a standard value for detection data individually because the photosensors cannot recognize each color toner mark when being overlaid. In this case, it becomes difficult to suppress the influence of error components during detection, e.g., detection timing and speed fluctuation of the transfer member during transfer, errors of detection timing relating to timing differences and the like. Furthermore, since such detection is accomplished based on the magnitude of the photosensor output, there is concern that sensor output may be reduced by soiling of the sensor and the like.